This invention relates to a furnace method and apparatus for burning carbonaceous fuel to produce energy in the form of heat.
One of the most important problems encountered in designing and operating furnaces is to control the amounts of nitrogen oxides in the furnace exhaust gases, since nitrogen oxides are an extremely serious air pollution problem. Substantial amounts of nitrogen oxides inevitably form when fuel and at least a stoichiometric amount of air are combusted at temperatures in excess of about 3300.degree.F. As used herein and in the appended claims, the term air means any gas or combination of gases containing oxygen available for combustion reactions, and the term stoichiometric amount of air means an amount of air which is theoretically sufficient for complete oxidation of all the combustible components in a given amount of fuel (e.g., to carbon dioxide and water). The term carbonaceous fuel means any fuel in which a substantial proportion of the fuel value is elemental carbon or carbon compounds containing carbon in combustible combination with other elements such as hydrogen.
One way of reducing the amounts of nitrogen oxides formed in a furnace is to lower the temperature in the furnace by mixing the fuel with an increased volume of effectively inert gas to produce a diluted fuel-air mixture. For reasons of thermal efficiency, it is almost always preferable to use no more air in the furnace than is required for complete combustion of the fuel (i.e., as close to the stoichiometric amount of air as possible). Any air in excess of the stoichiometric amount must be heated, generally from ambient temperature, to the temperature in the furnace and then exhausted into the atmosphere again, carrying with it whatever heat cannot be recovered for a useful purpose. The preferred source of inert gases for mixing with the fuel and air to lower combustion temperature are the final combustion effluent or stack gases of the furnace. These gases are warmer than ambient air, but substantially cooler than the gases leaving the combustion zone. Accordingly, a portion of the final combustion effluent gases of the furnace may be recycled to dilute the fuel and air supplied to the furnace to lower combustion temperatures in the furnace and help control the formation of nitrogen oxides.
Another way in which the formation of nitrogen oxides can be controlled is by conducting the combustion of the fuel in two or more successive stages. In the first stage, a mixture of fuel and an amount of air substantially less than the amount needed for complete combustion of the fuel (i.e., a non-stoichiometric mixture on the fuel-rich side) is thermally combusted to produce a gaseous effluent containing a substantial proportion of carbon monoxide. This effluent may also contain some uncombusted or partially combusted fuel. The temperature of this combustion (which is low relative to the stoichiometric combustion temperature) and particularly the insufficiency of air in this first thermal combustion stage substantially limit the formation of nitrogen oxides. Heat is withdrawn from the first stage effluent and that effluent is then mixed with additional air and thermally combusted in a subsequent thermal combustion stage or stages. The additional air is sufficient to make up the deficiency of air supplied to the first thermal combustion stage so that in the subsequent stage or stages all of the carbon monoxide in the first stage effluent is completely oxidized to carbon dioxide and any unburned or partially burned fuel in that effluent is completely oxidized to carbon dioxide and water. Although conditions in at least the last of the combustion stages must be sufficiently oxidizing to insure complete combustion of the fuel, less nitrogen oxides are produced than would be produced in a single-stage combustion system.
Although the foregoing systems decrease the formation of nitrogen oxides, these systems do not eliminate formation of nitrogen oxides and are, in addition, typically more difficult to operate and control. For example, in these systems it is frequently more difficult to achieve efficient thermal combustion reactions and avoid formation of substantial amounts of incomplete combustion products such as carbon monoxide and uncombusted hydrocarbons without the production of high concentrations of nitrogen oxides. Although only an insignificant amount of fuel value may be lost as a result of this incomplete combustion, the incomplete combustion products are another serious air pollution problem. To insure complete combustion of the fuel and for general ease of operation, furnaces are therefore frequently operated with substantially more air than is theoretically sufficient for complete combustion of the fuel supplied to the furnace. As mentioned above, however, excess air decreases thermal efficiency. In addition, there are practical limits on how far temperatures can be lowered in a conventional two-stage combustion furnace and still maintain stable combustion. This in turn limits the achievable reduction in nitrogen oxide formation.
In the furnaces described above, energy in the form of heat is withdrawn from the final combustion effluent for the purpose of furnace operation (e.g., to generate high pressure steam for use as a motive fluid in a steam turbine) until the temperature of the effluent is too low for further efficient transfer of heat for that purpose. Additional heat is then recovered from the effluent by conventional heat transfer to preheat the air supplied to the furnace. Frequently, however, more heat is available for recovery for air preheating than can be used without raising temperatures in the furnace to the point at which excessive amounts of nitrogen oxides would form. In most cases this excess heat must be wasted, although in some situations it may be possible to use some of it for other purposes, for example, to generate low pressure steam. The ability to use substantially all the available air preheat is an important advantage of furnaces constructed in accordance with the principles of this invention.
In view of the foregoing, it is an object of this invention to reduce the amount of atmospheric pollutants produced by furnaces burning carbonaceous fuels to produce thermal energy.
It is another object of this invention to increase the thermal efficiency of furnaces burning carbonaceous fuel to produce thermal energy.
It is yet another object of this invention to increase the efficiency of combustion in furnaces burning carbonaceous fuel to produce thermal energy, particularly in combustion with low production of nitrogen oxides.
In copending application Ser. No. 358,411, filed May 8, 1973, now U.S. Pat. No. 3,928,961, and incorporated herein by reference, there is disclosed the discovery of catalytically-supported, thermal combustion. According to this method, carbonaceous fuels can be combusted very efficiently at temperatures between about 1700.degree. and 3200.degree.F, for example, without the formation of substantial amounts of carbon monoxide or nitrogen oxides by a process designated catalytically-supported, thermal combustion. To summarize briefly what is discussed in greater detail in application Ser. No. 358,411, now U.S. Pat. No. 3,928,961, in conventional thermal combustion of carbonaceous fuels, a flammable mixture of fuel and air or fuel, air, and inert gases is contacted with an ignition source (e.g., a spark) to ignite the mixture. Once ignited, the mixture continues to burn without further support from the ignition source. Flammable mixtures of carbonaceous fuels normally burn at relatively high temperatures (i.e., normally well above 3300.degree.F). At these temperatures substantial amounts of nitrogen oxides inevitably form if nitrogen is present, as is always the case when air is the source of oxygen for the combustion reaction. Mixtures of fuel and air or fuel, air, and inert gases which would theoretically burn at temperatures below about 3300.degree.F are too fuel-lean to support a stable flame and therefore cannot be satisfactorily burned in a conventional thermal combustion system.
In conventional catalytic combustion, on the other hane, the fuel is burned at relatively low temperatures (typically in the range of from a few hundred degress Fahrenheit to approximately 1400.degree.F). Prior to the invention described in application Ser. No. 358,411, now U.S. Pat. No. 3,928,961, however, catalytic combustion was regarded as having limited value as a source of thermal energy. In the first place, conventional catalytic combustion proceeds relatively slowly so that impractically large amounts of catalyst would be required to produce enough combustion effluent gases to drive a turbine or to consume the large amounts of fuel required in most large furnace applications. In the second place, the reaction temperatures normally associated with conventional catalytic combustion are too low for efficient transfer of heat for many purposes, for example, transfer of heat to water in a steam boiler. Typically, catalytic combustion is also relatively inefficient, so that large amounts of carbon monoxide are produced or left uncombusted unless low space velocities in the catalyst are employed.
Catalytic combustion reactions follow the course of the graph shown in FIG. 1 of the accompanying drawing, to the extent of regions A through C in that Figure. This graph is a plot of reaction rate as a function of temperature for a given catalyst and set of reaction conditions. At relatively low temperatures (i.e., in region A of FIG. 1) the catalytic reaction rate increases exponentially with temperature. As the temperature is raised further, the reaction rate enters a transition zone (region B in the graph of FIG. 1) in which the rate at which the fuel and oxygen are being transferred to the catalytic surface begins to limit further increases in the reaction rate. As the temperature is raised still further, the reaction rate enters a so-called mass transfer limited zone (region C in the graph of FIG. 1) in which the reactants cannot be transferred to the catalytic surface fast enough to keep up with the catalytic surface reation and the reaction rate levels off regardless of further temperature increases. In the mass transfer limited zone, the reaction rate cannot be increased by increasing the activity of the catalyst because catalytic activity is not determinative of the reaction rate. Prior to the invention described in application Ser. No. 358,411, now U.S. Pat. No. 3,928,961, the only apparent way to increase the reaction rate in the mass transfer limited zone was to increase the mass transfer rate. However, this requires an increase in the pressure drop across the catalyst and consequently a substantial loss of energy. Sufficient pressure drop may not even be available to provide the desired reaction rate. Of course, more mass transfer can be effected, and hence more energy can always be produced by increasing the amount of catalyst surface. In many applications, however, this results in catalyst configurations of such size and complexity that the cost is prohibitive and the body of the catalyst is unwieldy. For example, in the case of gas turbine engines, the catalytic reactor might very well be larger than the engine itself.
As described in application Ser. No. 358,411, now U.S. Pat. No. 3,928,961, it has been discovered that it is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. In particular, it has been found that if the operating temperature of the catalyst is increased substantially into the mass transfer limited zone, the reaction rate again begins to increase rapidly with temperature (region D in the graph of FIG. 1). This is in apparent contradiction of the laws of mass transfer kinetics in catalytic reactions. The phenomenon may be explained by the fact that the temperature of the catalyst surface and the gas layer near the catalyst surface are above the instantaneous auto-ignition temperature of the mixture of fuel, air, and any inert gases (defined herein and in application Ser. No. 358,411 now U.S. Pat. No. 3,928,961 to mean the temperature at which the ignition lag of the mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion) and at a temperature at which thermal combustion occurs at a rate higher than the catalytic combustion rate. The fuel molecules entering this layer burn spontaneously without transport to the catalyst surface. As combustion progresses and the temperature increases, it is believed that the layer in which thermal combustion occurs becomes deeper. Ultimately, substantially all of the gas in the catalytic region is raised to a temperature at which thermal combustion occurs in virtually the entire gas stream rather than just near the surface of the catalyst. Once this stage is reached within the catalyst, the thermal reaction appears to continue even without further contact of the gas with the catalyst.
The foregoing is offered as a possible explanation only and is not to be construed as in any way limiting the present invention.
Among the unique advantages of the above-described combustion in the presence of a catalyst is the fact that mixtures of fuel and air which are too fuel-lean for ordinary thermal combustion can be burned efficiently. Since the temperature of combustion for a given fuel at any set of conditions (e.g., initial temperature and, to a lesser extent, pressure) is dependent largely on the proportions of fuel, of oxygen available for combustion, and of inert gases in the mixture to be burned, it becomes practical to burn mixtures which are characterized by much lower flame temperatures. In particular, carbonaceous fuels can be burned very efficiently and at thermal reaction rates at temperatures in the range from about 1700.degree. to about 3200.degree.F. At these temperatures very little nitrogen oxides are formed, if any, and indeed the reaction may be such as actually to decrease the amounts of nitrogen oxides present in the gases supplied to the reaction. In addition, because the combustion as described above is stable over a wide range of mixtures, it is possible to select or control reaction temperature over a correspondingly wide range by selecting or controlling the relative proportions of the gases in the mixture.
The combustion method as described in the copending application Ser. No. 358,411, now U.S. Pat. No. 3,928,961 involves essentially adiabatic combustion of a mixture of fuel and air or fuel, air, and inert gases in the presence of a solid oxidation catalyst operating at a temperature substantially above the instantaneous auto-ignition temperature of the mixture, but below a temperature which would result in any substantial formation of oxides of nitrogen under the conditions existing in the catalyst. The instantaneous auto-ignition temperature of the mixture is defined above. Essentially adiabatic combustion means in this case that the operating temperature of the catalyst does not differ by more than about 300.degree.f, more typically no more than about 150.degree.F, from the adiabatic flame temperature of the mixture due to heat losses from the catalyst.